Adhesives, often referred to as cements, glues or pastes are defined generally as substances capable of holding materials together by surface attachment. Adhesives may attach to surfaces and develop the internal or cohesive strength necessary to hold the materials together while cooling from liquid to solid state, while losing solvent, or during a chemical reaction. Many of the substances designated as adhesives may also be designated as caulking, potting, sealing, casting or encapsulating compounds when employed in a thick mass. In a more restrictive sense, to be termed an adhesive, a substance should be a liquid or a tack semi-solid, at least for a short period of time in order to contact and wet a surface, and should be used in a relatively thin layer to form a useful joint capable of transmitting stresses from one substrate to another. Thus, the term “adhesives”, as used in this specification and in the claims, includes cements, glues and pastes.
Solvent cementing is a process which thermoplastics, usually amorphous, are softened by the application of a suitable solvent or mixture of solvents, and then pressed together to effect a bond. Many thermoplastic substrate are easier to join effectively by solvent cements than by conventional adhesive bonding. Generally, a small amount of the same resin as that to be cemented is dissolved in a solvent to form the cement. The inclusion of the resin aids in gap filling, accelerates setting, and reduces shrinkage and internal stresses.
Solvents cements also have been utilized to bond different plastic materials to each other, but in such instances, the solvent must be a solvent for both plastics. Usually in such instances, a mixture of solvents is used. The solvent softens (dissolves) the surface of the resin to be bonded, and the surface becomes tacky. At this point, the surfaces are brought into contact with each other, often under pressure, and dried.
For many years, solvent cements have been for joining PVC (polyvinylchloride) and CPVC (chlorinated polyvinylchloride) plastic pipe and fittings. The amount of such plastic pipe used for conveying a variety of liquids is enormous. The major uses are drain, waste, vent sewer and potable water conveyance. Plastic pipe has increasingly displaced the traditional materials used for the same purpose such as copper, steel, galvanized metal, cast iron, lead and concrete asbestos pipe. Plastic pipe has become the material of choice in the home, municipal, and manufacturing industries.
The ASTM standard for PVC solvent cements is ASTM D-2564. According to this standard, such solvent cements contain a minimum of 10% PVC resin, inert fillers, and the remainder is one or more solvents from the group of tetrahydrofuran (THF), cyclohexanone (CYH), MEK, and acetone.
ASTM F-493 sets forth the requirements for CPVC solvent cements as containing a minimum of 10% CPVC resin, inert fillers, and the remainder is one or more solvents including THF, CYH, MEK and/or acetone.
Pipes and fittings of CPVC are used for applications where, in addition to high internal pressures of up to 5 bar, temperatures in the range from ambient to at least about 95° C. are encountered. Known adhesives for pipes and fittings of CPVC contain between 10 and 30% by weight CPVC in combination with suitable solvents, such as tetrahydrofuran, cyclohydrofuran, cyclohexanone, methyl ethyl ketone, N-methylpyrrolidone, methylene chloride, acetone, ethyl acetate and the like. The adhesives may contain consistency-generating constituents such as thickeners, thixotropic agents and the like. Known solvents have added various stabilizers, but have failed to completely resolve the problems resulting from the heat exposure of CPVC. Upon exposure to heat, CPVC both in the piping and structural components, and in the cement bonding together these components, degrades by loss of chlorine.
An additional problem encountered with solvent cements including chlorinated polymers and particularly CPVC, is that, depending on the storage temperature and particularly at elevated storage temperatures, the chlorinated polymer losses chlorine through heat-caused degradative processes. The loss of chlorine from CPVC due to heat degradation results in loss of desirable properties in the CPVC cement.
In addition to the resulting changes in the CPVC itself, due to the inevitable presence of moisture in the solvent cement (for example, as a result of exposure to atmospheric moisture or present as an impurity in the solvent), loss of chlorine can lead to container corrosion through the formation of hydrogen chloride. In order to store adhesives containing CPVC for prolonged periods without any risk of corrosion, the adhesives may be stored in containers made of materials which withstand the chemical attack of hydrogen chloride. However, such containers as glass or polyolefin are unsuitable for other reasons.
Thus, solvent cements for both PVC and CPVC continue to require the addition of stabilizers, particularly against the effects of heat. For the above reasons, this need is particularly acute for CPVC solvent cements since CPVC is specifically designed for use with hot liquids at elevated pressures. Because of the critical function of the cement or adhesives in maintaining the integrity of the piping system subjected to such conditions, there is a continuing need for improved solvent cement systems, particularly with respect to the stabilizers.